High-sensitivity pressure-sensing probe

ABSTRACT

A medical probe includes an insertion tube, having a longitudinal axis and having a distal end. A distal tip is disposed at the distal end of the insertion tube and is configured to be brought into contact with a body tissue. A joint couples the distal tip to the distal end of the insertion tube. A joint sensor, contained within the probe, senses a position of the distal tip relative to the distal end of the insertion tube. The joint sensor includes first and second subassemblies, which are disposed within the probe on opposite, respective sides of the joint and each include one or more magnetic transducers.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/868,733, filed Oct. 8, 2007, which is assigned to theassignee of the present patent application and whose disclosure isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to invasive medical devices, andspecifically to methods and devices for sensing displacement of a jointin a probe, such as a catheter, that is applied to the body of apatient.

BACKGROUND OF THE INVENTION

In some diagnostic and therapeutic techniques, a catheter is insertedinto a chamber of the heart and brought into contact with the innerheart wall. In such procedures, it is generally important that thedistal tip of the catheter engages the endocardium with sufficientpressure to ensure good contact. Excessive pressure, however, may causeundesired damage to the heart tissue and even perforation of the heartwall.

For example, in intracardiac radio-frequency (RF) ablation, a catheterhaving an electrode at its distal tip is inserted through the patient'svascular system into a chamber of the heart. The electrode is broughtinto contact with a site (or sites) on the endocardium, and RF energy isapplied through the catheter to the electrode in order to ablate theheart tissue at the site. Proper contact between the electrode and theendocardium during ablation is necessary in order to achieve the desiredtherapeutic effect without excessive damage to the tissue.

A number of patent publications describe catheters with integratedpressure sensors for sensing tissue contact. As one example, U.S. PatentApplication Publication 2007/0100332, whose disclosure is incorporatedherein by reference, describes systems and methods for assessingelectrode-tissue contact for tissue ablation. An electro-mechanicalsensor within the catheter shaft generates electrical signalscorresponding to the amount of movement of the electrode within a distalportion of the catheter shaft. An output device receives the electricalsignals for assessing a level of contact between the electrode and atissue.

SUMMARY OF THE INVENTION

The embodiments of the present invention that are described hereinbelowprovide novel apparatus and methods for sensing displacement of a joint,by generating and sensing magnetic fields using magnetic transducers,such as coils, on opposite sides of the joint. A disclosed embodimentrelates specifically to the use of this sort of sensing apparatus in aninvasive medical probe, in which the apparatus provides an indication ofpressure exerted on the tip of the probe. The principles of the presentinvention, however, are similarly useful in applications of other sortsthat require accurate sensing of joint displacement.

There is therefore provided, in accordance with an embodiment of thepresent invention a medical probe, including an insertion tube, having alongitudinal axis and having a distal end. A distal tip is disposed atthe distal end of the insertion tube and is configured to be broughtinto contact with a body tissue. A joint couples the distal tip to thedistal end of the insertion tube. A joint sensor, contained within theprobe, senses a position of the distal tip relative to the distal end ofthe insertion tube, the joint sensor including first and secondsubassemblies, which are disposed within the probe on opposite,respective sides of the joint and each include one or more magnetictransducers.

In some embodiments, the magnetic transducers includes coils, and thefirst subassembly includes a first coil having a first coil axisparallel to the longitudinal axis of the insertion tube, and the secondsubassembly includes two or more second coils in different, respectiveradial locations within a section of the probe that is spaced apartaxially from the first subassembly. In one embodiment, the second coilshave respective second coil axes that are parallel to the longitudinalaxis of the insertion tube. Additionally or alternatively, the two ormore second coils include at least three second coils, which aredisposed within an axial plane of the probe at different, respectiveazimuthal angles about the longitudinal axis.

In a disclosed embodiment, the joint sensor is configured to generate asignal indicative of an axial displacement and an orientation of thedistal tip relative to the distal end of the insertion tube. Typicallyone of the first and second subassemblies is coupled to be driven by anelectrical current to emit at least one magnetic field, and the other ofthe first and second subassemblies is coupled to output one or moresignals in response to the at least one magnetic field, wherein thesignals are indicative of the position of the distal tip relative to thedistal end of the insertion tube.

In one embodiment, the probe includes a position sensor for sensingposition coordinates of the probe relative to a frame of reference thatis separate from the probe. Additionally or alternatively, the distaltip includes an electrode, which is configured to make electricalcontact with the tissue.

In some embodiments, the joint includes a resilient member, which isconfigured to deform in response to pressure exerted on the distal tipwhen the distal tip engages the tissue. The resilient member may includea tubular piece of an elastic material having a helical cut therethroughalong a portion of a length of the piece.

There is also provided, in accordance with an embodiment of the presentinvention, apparatus for performing a medical procedure on a body of apatient. The apparatus includes a probe, which includes an insertiontube, having a longitudinal axis and having a distal end; a distal tip,which is disposed at the distal end of the insertion tube and isconfigured to be brought into contact with tissue of the body; a joint,which couples the distal tip to the distal end of the insertion tube;and a joint sensor, contained within the probe, for sensing a positionof the distal tip relative to the distal end of the insertion tube, thejoint sensor including first and second subassemblies, which aredisposed within the probe on opposite, respective sides of the joint andeach include one or more magnetic transducers. A processor is coupled toapply a current to one of the first and second subassemblies, therebycausing the one of the subassemblies to generate at least one magneticfield, and is coupled to receive and process one or more signals outputby the other of the first and second subassemblies responsively to theat least one magnetic field so as to detect changes in a position of thedistal tip relative to the distal end of the insertion tube.

In some embodiments, the apparatus includes a magnetic field generator,for generating a further magnetic field in a vicinity of the body, and aposition sensor in the probe for generating a position signal inresponse to the further magnetic field, wherein the processor is coupledto receive and process the position signal in order to computecoordinates of the probe relative to a frame of reference that isseparate from the probe. In a disclosed embodiment, the position sensorincludes at least one of the magnetic transducers in one of the firstand second subassemblies.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for sensing movement of a joint in anassembly having a longitudinal axis passing through the joint. Theapparatus includes first and second sensing subassemblies, which aredisposed within the assembly on opposite, respective sides of the jointand each include one or more magnetic transducers. A processor iscoupled to apply a current to one of the first and second assemblies,thereby causing the one of the assemblies to generate at least onemagnetic field, and is coupled to receive and process one or moresignals output by the other of the first and second assembliesresponsively to the at least one magnetic field so as to detect changesin a disposition of the joint.

There is further provided, in accordance with an embodiment of thepresent invention, a method for performing a medical procedure on tissuein a body of a patient. The method includes applying to the body aprobe, which includes an insertion tube and a distal tip, which iscoupled to a distal end of the insertion tube by a joint, and whichincludes a joint sensor, which is contained within the probe andincludes first and second subassemblies, which are disposed within theprobe on opposite, respective sides of the joint and each include one ormore magnetic transducers. The probe is advanced so that the distal tipengages and applies a pressure against the tissue, so as to cause achange in a position of the distal tip relative to the distal end of theinsertion tube. A current is applied to one of the first and secondsubassemblies, thereby causing the one of the subassemblies to generateat least one magnetic field. One or more signals output are received bythe other of the first and second subassemblies responsively to the atleast one magnetic field and are processed so as to detect the change inthe position of the distal tip.

In one embodiment, advancing the probe includes bringing an electrode onthe distal tip into electrical contact with the tissue. The method mayinclude applying electrical energy to the electrode so as to ablate aregion of the tissue that is engaged by the distal tip, wherein theposition of the distal tip relative to the distal end of the insertiontube changes in response to a pressure of the distal tip against thetissue, and wherein applying the electrical energy includes controllingapplication of the energy responsively to the pressure, as indicated bythe position of the distal tip, so that the electrical energy is appliedto the electrode when the pressure is within a desired range.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, pictorial illustration of a catheter-basedmedical system, in accordance with an embodiment of the presentinvention;

FIG. 2 is a schematic detail view showing the distal tip of a catheterin contact with endocardial tissue, in accordance with an embodiment ofthe present invention; and

FIG. 3 is a schematic, sectional view showing details of the distal endof a catheter, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The above-mentioned U.S. patent application Ser. No. 11/868,733describes a catheter whose distal tip is coupled to the distal end ofthe catheter insertion tube by a spring-loaded joint, which deforms inresponse to pressure exerted on the distal tip when it engages tissue. Amagnetic position sensing assembly within the probe, comprising coils onopposite sides of the joint, senses the position of the distal tiprelative to the distal end of the insertion tube. Changes in thisrelative position are indicative of deformation of the spring and thusgive an indication of the pressure.

Embodiments of the present invention that are described hereinbelowprovide a new design of the sensing assembly, which facilitates moreprecise measurement of tip movement. The configuration of the coils inthis new design permits precise sensing of very small deflections andcompressions of the joint connecting the catheter tip to the insertiontube. Therefore, the pressure on the tip can be measured with enhancedaccuracy, permitting the use a relatively stiffer spring in thecatheter, which makes the catheter more reliable and easier to maneuverin the body.

FIG. 1 is a schematic, pictorial illustration of a system 20 for cardiaccatheterization, in accordance with an embodiment of the presentinvention. System 20 may be based, for example, on the CARTO™ system,produced by Biosense Webster Inc. (Diamond Bar, Calif.). This systemcomprises an invasive probe in the form of a catheter 28 and a controlconsole 34. In the embodiment described hereinbelow, it is assumed thatcatheter 28 is used in ablating endocardial tissue, as is known in theart. Alternatively, the catheter may be used, mutatis mutandis, forother therapeutic and/or diagnostic purposes in the heart or in otherbody organs.

An operator 26, such as a cardiologist, inserts catheter 28 through thevascular system of a patient 24 so that a distal end 30 of the catheterenters a chamber of the patient's heart 22. The operator advances thecatheter so that the distal tip of the catheter engages endocardialtissue at a desired location or locations. Catheter 28 is typicallyconnected by a suitable connector at its proximal end to console 34. Theconsole may comprise a radio frequency (RF) generator, which supplieshigh-frequency electrical energy via the catheter for ablating tissue inthe heart at the locations engaged by the distal tip. Alternatively oradditionally, the catheter and system may be configured to perform othertherapeutic and diagnostic procedures that are known in the art.

Console 34 uses magnetic position sensing to determine positioncoordinates of distal end 30 of catheter 28 inside heart 22. For thispurpose, a driver circuit 38 in console 34 drives field generators 32 togenerate magnetic fields in the vicinity of the body of patient 24.Typically, the field generators comprise coils, which are placed belowthe patient's torso at known positions external to the patient. Thesecoils generate magnetic fields within the body in a predefined workingvolume that contains heart 22. A magnetic field sensor within distal end30 of catheter 28 (shown in FIG. 3) generates electrical signals inresponse to these magnetic fields. A signal processor 36 processes thesesignals in order to determine the position coordinates of the distalend, typically including both location and orientation coordinates. Thismethod of position sensing is implemented in the above-mentioned CARTOsystem and is described in detail in U.S. Pat. Nos. 5,391,199,6,690,963, 6,484,118, 6,239,724, 6,618,612 and 6,332,089, in PCT PatentPublication WO 96/05768, and in U.S. Patent Application Publications2002/0065455 A1, 2003/0120150 A1 and 2004/0068178 A1, whose disclosuresare all incorporated herein by reference.

Processor 36 typically comprises a general-purpose computer, withsuitable front end and interface circuits for receiving signals fromcatheter 28 and controlling the other components of console 34. Theprocessor may be programmed in software to carry out the functions thatare described herein. The software may be downloaded to console 34 inelectronic form, over a network, for example, or it may be provided ontangible media, such as optical, magnetic or electronic memory media.Alternatively, some or all of the functions of processor 36 may becarried out by dedicated or programmable digital hardware components.Based on the signals received from the catheter and other components ofsystem 20, processor 36 drives a display 42 to give operator 26 visualfeedback regarding the position of distal end 30 in the patient's body,as well as regarding displacement of the distal tip of the catheter, andstatus information and guidance regarding the procedure that is inprogress.

Alternatively or additionally, system 20 may comprise an automatedmechanism for maneuvering and operating catheter 28 within the body ofpatient 24. Such mechanisms are typically capable of controlling boththe longitudinal motion (advance/retract) of the catheter and transversemotion (deflection/steering) of the distal end of the catheter. Somemechanisms of this sort use DC magnetic fields for this purpose, forexample. In such embodiments, processor 36 generates a control input forcontrolling the motion of the catheter based on the signals provided bythe magnetic field sensor in the catheter. These signals are indicativeof both the position of the distal end of the catheter and of forceexerted on the distal end, as explained further hereinbelow.

FIG. 2 is a schematic sectional view of a chamber of a heart 22, showingdistal end 30 of catheter 28 inside the heart, in accordance with anembodiment of the present invention. The catheter comprises an insertiontube 50, which is typically inserted into the heart percutaneouslythrough a blood vessel, such as the vena cava or the aorta. An electrode56 on a distal tip 52 of the catheter engages endocardial tissue 58.Pressure exerted by the distal tip against the endocardium deforms theendocardial tissue locally, so that electrode 56 contacts the tissueover a relatively large area. In the pictured example, the electrodeengages the endocardium at an angle, rather than head-on. Distal tip 52therefore bends at an elastic joint 54 relative to the distal end ofinsertion tube 50 of the catheter. The bend facilitates optimal contactbetween the electrode and the endocardial tissue.

Because of the elastic quality of joint 54, the angle of bending and theaxial displacement of the joint are proportional to the pressure exertedby tissue 58 on distal tip 52 (or equivalently, the pressure exerted bythe distal tip on the tissue). Measurement of the bend angle and axialdisplacement thus gives an indication of this pressure. The pressureindication may be used by the operator of catheter 20 is ensuring thatthe distal tip is pressing against the endocardium firmly enough to givethe desired therapeutic or diagnostic result, but not so hard as tocause undesired tissue damage.

FIG. 3 is a schematic, sectional view of distal end 30 of catheter 28,showing details of the structure of the catheter in accordance with anembodiment of the present invention. Insertion tube 50 is connected todistal tip 52 by joint 54, as noted above. The insertion tube is coveredby a flexible, insulating material 62, such as Celcon®, Teflon®, orheat-resistant polyurethane, for example. The area of joint 54 iscovered, as well, by a flexible, insulating material, which may be thesame as material 62 or may be specially adapted to permit unimpededbending and compression of the joint. (This material is cut away in FIG.3 in order to expose the internal structure of the catheter.) Distal tip52 may be covered, at least in part, by electrode 56, which is typicallymade of a conductive material, such as a platinum/iridium alloy.Alternatively, other suitable materials may be used, as will be apparentto those skilled in the art. Further alternatively, for someapplications, the distal tip may be made without a covering electrode.The distal tip is typically relatively rigid, by comparison with theflexible insertion tube.

Joint 54 comprises a resilient coupling member 60. In this embodiment,the coupling member has the form of a tubular piece of an elasticmaterial, with a helical cut along a portion of its length. For example,the coupling member may comprise a superelastic alloy, such as nickeltitanium (Nitinol). The helical cut causes the tubular piece to behavelike a spring in response to forces exerted on distal tip 52. Furtherdetails regarding the fabrication and characteristics of this sort ofcoupling member are presented in U.S. patent application Ser. No.12/134,592, filed Jun. 6, 2008, which is assigned to the assignee of thepresent patent application and whose disclosure is incorporated hereinby reference. Alternatively, the coupling member may comprise a coilspring or any other suitable sort of resilient component with thedesired flexibility and strength characteristics.

The stiffness of coupling member 60 determines the range of relativemovement between tip 52 and insertion tube 50 in response to forcesexerted on the distal tip. Such forces are encountered when the distaltip is pressed against the endocardium during an ablation procedure. Thedesired pressure for good electrical contact between the distal tip andthe endocardium during ablation is on the order of 20-30 grams. Thecoupling member is configured to permit axial displacement (i.e.,lateral movement along the axis of catheter 28) and angular deflectionof the distal tip in proportion to the pressure on the tip. Measurementof the displacement and deflection by processor 36 gives an indicationof the pressure and thus helps to ensure that the correct pressure isapplied during ablation.

A joint sensing assembly, comprising coils 64, 66, 68 and 70 withincatheter 28, provides accurate reading of the position of distal tip 52relative to the distal end of insertion tube 50, including axialdisplacement and angular deflection. These coils are one type ofmagnetic transducer that may be used in embodiments of the presentinvention. A “magnetic transducer,” in the context of the present patentapplication and in the claims, means a device that generates a magneticfield in response to an applied electrical current and/or outputs anelectrical signal in response to an applied magnetic field. Although theembodiments described herein use coils as magnetic transducers, othertypes of magnetic transducers may be used in alternative embodiments, aswill be apparent to those skilled in the art.

The coils in catheter 28 are divided between two subassemblies onopposite sides of joint 54: One subassembly comprises coil 64, which isdriven by a current via a cable 74 from console 34 to generate amagnetic field. This field is received by a second subassembly,comprising coils 66, 68 and 70, which are located in a section of thecatheter that is spaced axially apart from coil 64. (The term “axial,”as used in the context of the present patent application and in theclaims, refers to the direction of the longitudinal axis of distal end30 of catheter 28, which is identified as the Z-direction in FIG. 3. Anaxial plane is a plane perpendicular to this longitudinal axis, and anaxial section is a portion of the catheter contained between two axialplanes.) Coils 66, 68 and 70 emit electrical signals in response to themagnetic field generated by coil 64. These signals are conveyed by cable74 to processor 36, which processes the signals in order to measure theaxial displacement and angular deflection of joint 54.

Coils 66, 68 and 70 are fixed in catheter 28 at different radiallocations. (The term “radial” refers to coordinates relative to thecatheter axis, i.e., coordinates in an X-Y plane in FIG. 3.)Specifically, in this embodiment, coils 66, 68 and 70 are all located inthe same axial plane at different azimuthal angles about the catheteraxis. For example, the three coils may be spaced azimuthally 1200 apartat the same radial distance from the axis.

The axes of coils 64, 66, 68 and 70 are parallel to the catheter axis(and thus to one another, as long as joint 54 is undeflected).Consequently, coils 66, 68 and 70 will output strong signals in responseto the field generated by coil 64, and the signals will vary stronglywith the distances of coils 66, 68 and 70 from coil 64. (Alternatively,the axis of coil 64 and/or coils 66, 68 and 70 may be angled relative tothe catheter axis, as long as the coil axes have a sufficient parallelcomponent in order to give substantial signals.) Angular deflection oftip 52 will give rise to a differential change in the signals output bycoils 66, 68 and 70, depending on the direction and magnitude ofdeflection, since one or two of these coils will move relatively closerto coil 64. Compressive displacement of the tip will give rise to anincrease in the signals from all of coils 66, 68 and 70.

Processor 36 analyzes the signals output by coils 66, 68 and 70 in orderto measure the deflection and displacement of joint 54. The sum of thechanges in the signals gives a measure of the compression, while thedifference of the changes gives the deflection. The vector direction ofthe difference gives an indication of the bend direction. A suitablecalibration procedure may be used to measure the precise dependence ofthe signals on deflection and displacement of the joint.

Various other configurations of the coils in the sensing subassembliesmay also be used, in addition to the configuration shown and describedabove. For example, the positions of the subassemblies may be reversed,so that that field generator coil is on the proximal side of joint 54,and the sensor coils are in the distal tip. As another alternative,coils 66, 68 and 70 may be driven as field generators (using time-and/or frequency-multiplexing to distinguish the fields), while coil 64serves as the sensor. The sizes and numbers of the coils in FIG. 3 areshown only by way of example, and larger or smaller numbers of coils maysimilarly be used, in various different positions, so long as one of thesubassemblies comprises at least two coils, in different radialpositions, to allow differential measurement of joint deflection.

Prior calibration of the relation between pressure on tip 52 andmovement of joint 54 may be used by processor 36 in translating the coilsignals into terms of pressure. By virtue of the combined sensing ofdisplacement and deflection, this pressure sensing system reads thepressure correctly regardless of whether the electrode engages theendocardium head-on or at an angle. The pressure reading is insensitiveto temperature variations and free of drift, unlike piezoelectricsensors, for example. Because of the high sensitivity to joint motionthat is afforded by the arrangement of coils 64, 66, 68 and 70 that isshown in FIG. 3, processor 36 can measure small displacements anddeflections with high precision. Therefore, coupling member 60 can bemade relatively stiff, and processor 36 will still be able to sense andmeasure accurately the pressure on tip 52. The stiffness of the couplingmember makes it easier for the operator to maneuver and control thecatheter.

One or more of coils 64, 66, 68 and 70 may also be used to outputsignals in response to the magnetic fields generated by field generators32, and thus serve as position sensing coils. Processor 36 processesthese signals in order to determine the coordinates (position andorientation) of distal end 30 in the external frame of reference that isdefined by the field generators. Additionally or alternatively, one ormore further coils 72 (or other magnetic sensors) may be deployed in thedistal end of the catheter for this purpose. The position sensing coilsin distal end 30 of catheter 28 enable console 34 to output both thelocation and orientation of the catheter in the body and thedisplacement and deflection of tip 52, as well as the pressure on thetip.

Although the operation of a magnetic position sensing assembly and itsuse in sensing pressure are described above in the context ofcatheter-based ablation, the principles of the present invention maysimilarly be applied in other applications that require accurate sensingof the movement of a joint, and particularly in therapeutic anddiagnostic applications that use invasive probes, both in the heart andin other organs of the body. As one example, the devices and techniquesfor position and pressure sensing that are implemented in system 20 maybe applied, mutatis mutandis, in guiding and controlling the use of acatheter insertion sheath. If the position of the sheath is not properlycontrolled and excessive force is used in its insertion, the sheath mayperforate the heart wall or vascular tissue. This eventuality can beavoided by sensing the position of and pressure on the distal tip of thesheath. In this regard, the term “distal tip” as used herein should beunderstood to include any sort of structure at the distal end of a probethat may be bent and/or displaced relative to the main body of theprobe.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

1. A medical probe, comprising: an insertion tube, having a longitudinalaxis and having a distal end; a distal tip, which is disposed at thedistal end of the insertion tube and is configured to be brought intocontact with a body tissue; a joint, which couples the distal tip to thedistal end of the insertion tube; and a joint sensor, contained withinthe probe, for sensing a position of the distal tip relative to thedistal end of the insertion tube, the joint sensor comprising first andsecond subassemblies, which are disposed within the probe on opposite,respective sides of the joint and each comprise one or more magnetictransducers.
 2. The probe according to claim 1, wherein the magnetictransducers comprises coils, and wherein the first subassembly comprisesa first coil having a first coil axis parallel to the longitudinal axisof the insertion tube, and wherein the second subassembly comprises twoor more second coils in different, respective radial locations within asection of the probe that is spaced apart axially from the firstsubassembly.
 3. The probe according to claim 2, wherein the second coilshave respective second coil axes that are parallel to the longitudinalaxis of the insertion tube.
 4. The probe according to claim 2, whereinthe two or more second coils comprise at least three second coils. 5.The probe according to claim 4, wherein the at least three second coilsare disposed within an axial plane of the probe at different, respectiveazimuthal angles about the longitudinal axis.
 6. The probe according toclaim 1, wherein the joint sensor is configured to generate a signalindicative of an axial displacement and an orientation of the distal tiprelative to the distal end of the insertion tube.
 7. The probe accordingto claim 6, wherein one of the first and second subassemblies is coupledto be driven by an electrical current to emit at least one magneticfield, and the other of the first and second subassemblies is coupled tooutput one or more signals in response to the at least one magneticfield, wherein the signals are indicative of the position of the distaltip relative to the distal end of the insertion tube.
 8. The probeaccording to claim 1, and comprising a position sensor for sensingposition coordinates of the probe relative to a frame of reference thatis separate from the probe.
 9. The probe according to claim 1, whereinthe distal tip comprises an electrode, which is configured to makeelectrical contact with the tissue.
 10. The probe according to claim 1,wherein the joint comprises a resilient member, which is configured todeform in response to pressure exerted on the distal tip when the distaltip engages the tissue.
 11. The probe according to claim 10, wherein theresilient member comprises a tubular piece of an elastic material havinga helical cut therethrough along a portion of a length of the piece. 12.Apparatus for performing a medical procedure on a body of a patient, theapparatus comprising: a probe, which comprises: an insertion tube,having a longitudinal axis and having a distal end; a distal tip, whichis disposed at the distal end of the insertion tube and is configured tobe brought into contact with tissue of the body; a joint, which couplesthe distal tip to the distal end of the insertion tube; and a jointsensor, contained within the probe, for sensing a position of the distaltip relative to the distal end of the insertion tube, the joint sensorcomprising first and second subassemblies, which are disposed within theprobe on opposite, respective sides of the joint and each comprise oneor more magnetic transducers; and a processor, which is coupled to applya current to one of the first and second subassemblies, thereby causingthe one of the subassemblies to generate at least one magnetic field,and which is coupled to receive and process one or more signals outputby the other of the first and second subassemblies responsively to theat least one magnetic field so as to detect changes in a position of thedistal tip relative to the distal end of the insertion tube.
 13. Theapparatus according to claim 12, wherein the magnetic transducerscomprise coils, and wherein the first subassembly comprises a first coilhaving a first coil axis parallel to the longitudinal axis of theinsertion tube, and wherein the second subassembly comprises two or moresecond coils in different, respective radial locations within a sectionof the probe that is spaced apart axially from the first subassembly.14. The apparatus according to claim 12, wherein the changes in theposition of the distal tip detected by the processor comprise an axialdisplacement of the distal tip and a deflection of the distal tiprelative to the distal end of the insertion tube.
 15. The apparatusaccording to claim 12, wherein the joint comprises a resilient member,which is configured to deform in response to pressure exerted on thedistal tip when the distal tip engages the tissue.
 16. The apparatusaccording to claim 15, wherein the processor is configured to generate,responsively to the detected changes in the position, an output that isindicative of the pressure exerted on the distal tip.
 17. The apparatusaccording to claim 12, and comprising a magnetic field generator, forgenerating a further magnetic field in a vicinity of the body, and aposition sensor in the probe for generating a position signal inresponse to the further magnetic field, wherein the processor is coupledto receive and process the position signal in order to computecoordinates of the probe relative to a frame of reference that isseparate from the probe.
 18. The apparatus according to claim 17,wherein the position sensor comprises at least one of the magnetictransducers in one of the first and second subassemblies.
 19. Apparatusfor sensing movement of a joint in an assembly having a longitudinalaxis passing through the joint, the apparatus comprising: first andsecond sensing subassemblies, which are disposed within the assembly onopposite, respective sides of the joint and each comprise one or moremagnetic transducers; and a processor, which is coupled to apply acurrent to one of the first and second assemblies, thereby causing theone of the assemblies to generate at least one magnetic field, and whichis coupled to receive and process one or more signals output by theother of the first and second assemblies responsively to the at leastone magnetic field so as to detect changes in a disposition of thejoint.
 20. The apparatus according to claim 19, wherein the magnetictransducers comprise coils, and wherein the first subassembly comprisesa first coil having a first coil axis parallel to the longitudinal axisof the insertion tube, and wherein the second subassembly comprises twoor more second coils in different, respective radial locations within asection of the assembly that is spaced apart axially from the firstsubassembly.
 21. The apparatus according to claim 19, wherein theprocessor is configured to detect, by processing the one or moresignals, an axial compression of the joint and an angular deflection ofthe joint.
 22. The apparatus according to claim 19, wherein the jointcomprises a resilient member, which is configured to deform in responseto pressure exerted on the assembly, and wherein the processor isconfigured to generate, responsively to the detected changes in thedisposition, an output that is indicative of the pressure exerted on theassembly.
 23. A method for performing a medical procedure on tissue in abody of a patient, the method comprising: applying to the body a probe,which comprises an insertion tube and a distal tip, which is coupled toa distal end of the insertion tube by a joint, and which comprises ajoint sensor, which is contained within the probe and comprises firstand second subassemblies, which are disposed within the probe onopposite, respective sides of the joint and each comprise one or moremagnetic transducers; advancing the probe so that the distal tip engagesand applies a pressure against the tissue, so as to cause a change in aposition of the distal tip relative to the distal end of the insertiontube; applying a current to one of the first and second subassemblies,thereby causing the one of the subassemblies to generate at least onemagnetic field; and receiving and processing one or more signals outputby the other of the first and second subassemblies responsively to theat least one magnetic field so as to detect the change in the positionof the distal tip.
 24. The method according to claim 23, wherein themagnetic transducers comprise coils, and wherein the first subassemblycomprises a first coil having a first coil axis parallel to alongitudinal axis of the insertion tube, and wherein the secondsubassembly comprises two or more second coils in different, respectiveradial locations within a section of the probe that is spaced apartaxially from the first subassembly.
 25. The method according to claim23, wherein processing the one or more signals comprises detecting anaxial displacement and an orientation of the distal tip relative to thedistal end of the insertion tube.
 26. The method according to claim 23,wherein the joint comprises a resilient member, which is configured todeform in response to the pressure on the distal tip, and whereinprocessing the one or more signals comprises generating, responsively tothe detected change in the position, an indication of the pressureexerted on the distal tip.
 27. The method according to claim 23, andcomprising generating a further magnetic field in a vicinity of thebody, and sensing a position signal output by one of the first andsecond subassemblies in response to the further magnetic field in orderto compute coordinates of the probe relative to a frame of referencethat is separate from the probe.
 28. The method according to claim 23,wherein advancing the probe comprises bringing an electrode on thedistal tip into electrical contact with the tissue.
 29. The methodaccording to claim 28, and comprising applying electrical energy to theelectrode so as to ablate a region of the tissue that is engaged by thedistal tip.
 30. The method according to claim 29, wherein the positionof the distal tip relative to the distal end of the insertion tubechanges in response to a pressure of the distal tip against the tissue,and wherein applying the electrical energy comprises controllingapplication of the energy responsively to the pressure, as indicated bythe position of the distal tip, so that the electrical energy is appliedto the electrode when the pressure is within a desired range.